A delicious heat wave

Dr. E. Kirsten Peters

POSTEDNovember 17, 2009 9:39 p.m.

My 84-year old mother bent over the cookbook one day recently and read aloud to me as I wolfed down a chicken sandwich I'd made at lunchtime. The reading was a lesson in how to make a traditional — and very fine as it turned out — pork roast. Personally, I suspect it would be morally responsible to live as a vegetarian, and certainly good for my family's health and for the nation's medical-care bills. But I'm a sinner, and my kitchen produces meat and poultry meals on a daily basis. My mother read aloud the part of the recipe we both knew well concerning what happens when you take a beef or pork roast out of the oven. For a good bit of time, the meat will continue to cook as it rests on your countertop. And, indeed, the temperature inside a roast actually rises for about 5 -10 minutes after you take it out of the oven. What's up with that? While it's cooking in the oven, the roast experiences a moving "wave" of intense heat that's coming from the oven into the roast from all its surfaces. At first, only the outermost smidgen of the roast is warmed. Then it becomes hot and a smidgen more inside the roast becomes warmed — and so on. Over a couple of hours, the wave of heat that started on the surface of the roast has penetrated into it, further and further. When you take the roast out of the oven, that wave of moving heat starts to collapse. But the decline takes a bit of time, and the wave is still moving inward. So the inner portion of the roast can and does warm further. Geologists love heat waves on a much bigger and slower scale. Here's why: Last summer, the solid rock and soil around your home was warmer than it is now. That wave of warmth went down into the Earth all summer long, growing like the heat-wave in the roast in the oven. The wave that's down under our feet got launched going into the Earth, and it will continue to move downward. By mid-winter, it will be about 60 feet below the surface. Yes: last July is really down there, about six times the depths of my basement's floor. And the heat wave from the summer of 2008 is down in the Earth, about 120 feet deep.I like that idea very much. I'm not sure why, but the history of old heat waves is pretty cool. Yet it's also true the waves are getting a whole lot smaller in size. Really smaller, and rapidly so. This winter, last summer's heat wave will be only 0.002 as big as it was at its peak. And the summer of 2008 warmth will be reduced to 4 parts in a million of what it was. The numbers just mentioned came to me from Fred Gittes, a physics faculty member here at Washington State University. Because he's not a rock-head, but a clear-thinking physicist, he worked out the mathematical details as formulas and graphs for my pork roast, just for fun. But he had to use basic figures for the conduction of heat and the like assuming that the pork roast would behave as water does. That's because physicists and geologists, at least, don't have a completely detailed picture of just how heat gets into the meat in your oven. That's clearly our loss. The roast that inspired this train of thought is long since gone. The next roast in my house is coming up shortly, but it's going to be a fancy "crown" roast with a big, gaping hole in the middle. But bear with me and later in the fall I'll take some measurements on a solid beef roast and a stuffed turkey — and report back on the heating-in-the-center effect. No matter what the calculations of physicists may be, as a rock-head I trust the simple measurements of thermometers. Luckily, Fred is an easy-going fellow who takes an interest in this project and agrees with me in some ways. "All food experiments are good," he said. — Dr. E. Kirsten Peters is a native of the rural Northwest, but was trained as a geologist at Princeton and Harvard. Questions about science or energy for future Rock Docs can be sent to epeters@wsu.edu. This column is a service of the College of Sciences at Washington State University.